Encapsulation or sequestration of hydrophobic molecules for transport within biological systems has been a topic of wide interest and research. Hydrophobic drugs comprise a substantial proportion of all pharmaceutical compounds in use today. These drugs have limited solubility in water. For applications where precise dosing is required, oral delivery can lead to variable bioavailability. In some cases, parenteral administration is the preferred route. In some cases, hydrophobic drugs may be coaxed into aqueous solutions by changing the solution pH or by adding appropriate salts. In other cases, small amounts of solubilizing excipients such as dextrins or lipids are sufficient. However, for many compounds that are yet more difficult to dissolve, other excipient strategies are required. These frequently involve formulations formed from surfactants and non-aqueous solvents. Non-ionic surfactants such as Cremophor EL and Polysorbate-80 are commonly used for parental formulations, but can induce negative side-effects including anaphylactic hypersensitivity and neurotoxicity. Non-aqueous solvents have potential to cause hemolysis and in the case of oils, pulmonary microembolisms. For injectable formulations, neutral pH, isotonic solutions in water are preferred. New drug delivery systems that bypass these problems promise to give rise to next generation formulations and indeed have been gradually making their ways to the clinic. However, many drug delivery systems described to date are themselves formed with excipients in relatively large quantities that themselves may carry side effects as well as unknown long-term safety profiles. Thus, the mass or molar drug-to-excipient ratios of current nanoparticulate delivery systems may not be significantly better compared to surfactant solutions and typically are close to 1:10 mass ratio (drug:excipient). Clinical adoption of alternative drug delivery systems has been limited due to both formulation complexities and low drug-loading capacities.
Hydrophobic molecules are also often used as imaging agents. Imaging of the gastrointestinal tract is used in diagnostics. However, modalities based on X-ray radiation, magnetic resonance, and ultrasound suffer from limitations with respect to safety, accessibility or lack of adequate contrast. For example, functional intestinal imaging of dynamic gut processes has not been practical using existing approaches.